Container with thermally fused double-seamed or crimp-seamed metal end

ABSTRACT

A container includes a metal end applied and sealed to an all-thermoplastic container body by a crimp-seaming or double-seaming operation. The metal end has an outer curl joined to a chuck wall that extends down from the curl. One or both of the inner surface of the container side wall and the outer surface of the chuck wall has/have a heat-sealable material thereon. The metal end is crimp-seamed or double-seamed to the container body and the heat-sealable material(s) are heated to soften or melt such that the interface between the chuck wall and the side wall is fused. The interface is oriented along a direction relative to internal pressure exerted on the metal end such that stress on the interface caused by the internal pressure is predominantly shear stress.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/638,420, filed Mar. 4, 2015, which is a continuation of U.S. patentapplication Ser. No. 13/224,651, filed Sep. 2, 2011, each of which ishereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present disclosure relates to containers, particularly to containershaving one or two metal ends applied to one or both ends of thecontainer body and crimp-seamed or double-seamed onto the containerbody, and most particularly to such containers used for retortprocessing and in which the container body is formed substantially ofthermoplastic.

Traditionally, retort containers have been constructed substantially ofmetal. For many decades the standard retort food containers have beenthree-piece or two-piece metal cans. In a three-piece metal can, a metalcan body is closed by a pair of metal ends that are typicallydouble-seamed onto the ends of the can body. A two-piece metal caneliminates one of the metal ends because the can body is a deep-drawnbody with an integral bottom wall. The metal ends of such typical retortcontainers have an outer peripheral portion forming a “curl” thatreceives the end of the can body, and after the end is applied the curland the wall of the can body are rolled up together to form a doubleseam. This construction has the great advantage that it readilywithstands retort processing without the seams being compromised,because the plastically deformed metal of the can body in the seam areatends to hold its deformed shape despite the high pressure andtemperature during retort.

More recently there has been a desire to construct retort containersthat use less metal, motivated by the potential cost reduction andimproved aesthetics that such a construction can offer. The developmentdescribed in the present disclosure at least in some aspects is aimed ataddressing this desire.

BRIEF SUMMARY OF THE INVENTION

In particular, the present disclosure describes a retort containerhaving one or two metal ends attached to a substantially thermoplasticcontainer body in such a way that there is an improvement in blow-offresistance when the inside of the container is pressurized relative tooutside ambient pressure for any reason (e.g., during retort processing,or as a result of changes in altitude of the container, such as when acontainer is filled and sealed at sea level and is subsequentlytransported to a high-altitude location).

In accordance with the invention in one embodiment, a retort containercomprises:

-   -   a container body constructed substantially of thermoplastic and        having a side wall extending about a container body axis, the        side wall having a lower end and an upper end, the upper end        defining an upper edge that extends about a top opening of the        container body, the side wall having an inner surface and an        outer surface;    -   a metal end closing the top opening of the container body, the        metal end having at least a metal layer and comprising a central        portion and an outer peripheral portion extending generally        radially outwardly from the central portion and extending        circumferentially about the central portion, the peripheral        portion having a radially outer part and a radially inner part,        a first heat-sealable material being present on at least one        of (a) a lower surface of at least the peripheral portion of the        metal end and (b) the inner surface of the side wall adjacent        the upper end thereof, the radially outer part of the peripheral        portion defining a curl, the radially inner part of the        peripheral portion defining a chuck wall that extends generally        downward from the curl and has a radially outer surface forming        an interface with the inner surface of the side wall of the        container body;    -   a seam connecting the metal end to the upper end of the side        wall, the seam having the curl of the metal end and the upper        end of the side wall interlocked; and    -   the interface between the chuck wall and the side wall being        fused by heat-sealing of the first heat-sealable material        between the radially outer surface of the chuck wall and the        inner surface of the side wall, the interface being oriented        along a direction such that stress on the interface caused by        internal pressure inside the container exerted on the metal end        is predominantly shear stress.

The retort container in accordance with a further embodiment can includea second heat-sealable material present on the other of (a) the lowersurface of at least the peripheral portion of the metal end and (b) theinner surface of the side wall adjacent the upper end thereof. In thisembodiment, the second heat-sealable material and the firstheat-sealable material are in contact with each other and are thermallyfused together such that the interface between the chuck wall and theside wall is fused. Advantageously, but not essentially, the secondheat-sealable material and the first heat-sealable material arethermally fused together in the seam as well.

Heat-sealable materials useful in the practice of the present inventioncan comprise any known heat-sealable materials. The metal end can havean interior coating, and optionally an exterior coating as well.

The seam between the metal end and the side wall can be a crimp seam ora double seam. In the case of a double seam, the upper end of the sidewall forms a body hook and the curl of the metal end forms an end hookinterlocked with the body hook.

The container body can be made and configured in various ways. Forexample, the container body can be a blow-molded, thermoformed, orinjection-molded container body having a bottom wall integrally joinedto the side wall. Alternatively, the container body can be an extrudedcontainer body having an open lower end, in which case the lower end isclosed by a second metal end similar to that closing the top end.

In some embodiments, the metal end is an easy-open end having aseverable panel defined by a score line in the metal layer.Alternatively, the metal end can be a sanitary end, or the metal end cancomprise a membrane sealed to an annular metal ring.

The present disclosure also describes methods for making containers. Inone embodiment a method for making a container comprises the steps of:

-   -   providing a container body constructed substantially of        thermoplastic and having a side wall extending about a container        body axis, the side wall having a lower end and an upper end,        the upper end defining an upper edge that extends about a top        opening of the container body, the side wall having an inner        surface and an outer surface;    -   providing a metal end for closing the top opening of the        container body, the metal end having at least a metal layer and        comprising a central portion and an outer peripheral portion        extending generally radially outwardly from the central portion        and extending circumferentially about the central portion, the        peripheral portion having a radially outer part and a radially        inner part, a first heat-sealable material being present on one        of (a) a lower surface of at least the peripheral portion of the        metal end and (b) the inner surface of the side wall adjacent        the upper end thereof, the radially outer part of the peripheral        portion defining a curl, the radially inner part of the        peripheral portion defining a chuck wall that extends generally        downward from the curl and has a radially outer surface;    -   applying the metal end to the container body such that the metal        end closes the top opening and the radially outer surface of the        chuck wall and the inner surface of the side wall have an        intimately contacting interface therebetween;    -   forming a seam connecting the metal end to the upper end of the        side wall, the seam being formed by interlocking the curl of the        metal end with the upper end of the side wall;    -   after formation of the seam is completed, heating the first        heat-sealable material to a temperature sufficient to cause the        first heat-sealable material to be softened or melted and to wet        the radially outer surface of the chuck wall and the inner        surface of the side wall; and    -   allowing the first heat-sealable material to cool and harden        such that the interface between the chuck wall and the side wall        is fused, the interface being oriented along a direction such        that stress on the interface caused by internal pressure inside        the container exerted on the metal end is predominantly shear        stress.

The step of forming a seam can comprise forming a crimp seam, or it cancomprise forming a double seam by rolling the curl of the metal end andthe upper end of the side wall together so as to form the upper end ofthe side wall into a body hook and to form the curl into an end hook andto interlock the body hook and the end hook.

The heating step can be carried out in any of various ways, includingheating by conduction, heating by induction, frictional heating, etc.

In some embodiments the method can further comprise providing a secondheat-sealable material present on the other of (a) the lower surface ofat least the peripheral portion of the metal end and (b) the innersurface of the side wall adjacent the upper end thereof. Thus, the metalend and the side wall both have respective heat-sealable materialsthereon. The method entails placing the second heat-sealable materialand the first heat-sealable material in contact with each other at theinterface between the chuck wall and the side wall, and heating thefirst and second heat-sealable materials to a temperature sufficient tocause the first and second heat-sealable materials to be softened ormelted and to flow together, after which cooling of the first and secondheat-sealable materials is allowed to occur so as to fuse the chuck wallto the inner surface of the side wall.

The second heat-sealable material and the first heat-sealable materialcan be thermally fused together in the seam as well.

The container body can be made by various processes, including, forexample, blow molding, thermoforming, or extrusion. In the case of ablow-molded or thermoformed container body, the container body includesa bottom wall integrally joined to the side wall. In the case of anextruded container body, the container body has an open lower end andtherefore a second metal end is attached to the lower end. The secondmetal end and its attachment to the container body can be substantiallyidentical to the first metal end and its attachment to the containerbody.

The method can further comprise the steps of filling the container witha food product prior to the step of applying the metal end to thecontainer body, and, after the interface between the chuck wall and theside wall is fused, retorting the container. During the retorting stepthe thermoplastic container body is radially unconstrained such that thecontainer body is allowed to expand radially as internal pressure isexerted on the side wall. Notably, the container body is free of anyspecial expansion panels, whereby the radial expansion of the containerbody occurs substantially uniformly about a circumference of thecontainer body.

In some embodiments, the chuck wall extends at a non-zero acute anglerelative to a longitudinal axis of the container body and is configuredsuch that a lower end of the chuck wall is smaller in diameter than theinner surface of the side wall, while an upper end of the chuck wall islarger in diameter than the inner surface of the side wall. The step ofapplying the metal end to the container body results in the side wall ofthe container body moving relatively upward from the lower end to theupper end of the chuck wall such that an interference fit is createdbetween the chuck wall and the side wall, thereby creating theintimately contacting interface therebetween.

During the heating step there is a substantial absence of externalpressure exerted on the chuck wall and side wall; rather, pressurebetween the chuck wall and side wall comes from the interference fitthat already exists between them when the end is applied and seamed tothe side wall. Thus, there is no need for sealing jaws to createpressure during the heating step in order to form a secure thermal bondbetween the metal end and the container body. Indeed, in someembodiments the heating step can be carried out with induction heatingin which there can be an absence of contact between the induction tooland the metal end.

The present disclosure also describes a packaging and retorting processfor packaging and sterilizing a food product. In one embodiment, amethod for packaging and retort-processing a food product comprises thesteps of:

-   -   providing a container assembly that includes a substantially        thermoplastic container body having a side wall and further        includes an end wall closing a lower end of the container body,        an opposite upper end of the container body being open;    -   providing a metal end having at least a metal layer and        comprising a central portion and an outer peripheral portion        extending generally radially outwardly from the central portion        and extending circumferentially about the central portion, the        peripheral portion having a curl and a chuck wall that extends        generally downward and radially inwardly from the curl;    -   providing at least one heat-sealable material on at least one        of (a) a lower surface of the peripheral portion of the metal        end and (b) an inner surface of the container body adjacent the        upper end thereof;    -   placing the food product into the container assembly through the        open end of the container body;    -   forming a crimp seam between the metal end onto the container        body to close the open end thereof, the forming step causing the        side wall of the container body to be compressed between the        chuck wall on an inner side of the side wall and a permanently        deformed portion of the metal end formed by deforming the curl        on an outer side of the side wall;    -   thermally fusing the metal end to the container body by causing        the heat-sealable material(s) to be melted where the metal end        compresses the side wall and then allowing the melted        heat-sealable material(s) to cool and harden, thereby completing        a filled container; and    -   retort-processing the filled container to sterilize the food        product and interior of the container.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a diagrammatic depiction of several steps of a process formaking containers in accordance with one embodiment of the invention;

FIG. 2 is a photomicrograph of a sectioned container in the region ofthe metal end's seam with the container body, in accordance with anembodiment of the invention;

FIG. 3 is a schematic depiction of a further step of the process formaking containers, wherein the seamed metal end is induction sealed tothe container body, in accordance with an embodiment of the invention;

FIG. 4 is a cross-sectional view through the region of the metal end'sseam with the container body, after the induction sealing step, inaccordance with an embodiment of the invention;

FIG. 5 is a cross-sectional view through the region of the metal end'sseam with the container body, after the induction sealing step, inaccordance with another embodiment of the invention;

FIG. 6 is a cross-sectional view through the region of the metal end'sseam with the container body, after the induction sealing step, inaccordance with a further embodiment of the invention;

FIG. 7 is a diagrammatic depiction of a test retort apparatus fortesting containers made in accordance with embodiments of the invention;

FIG. 8 is a graph of differential pressure (=can pressure−retortpressure) and retort temperature versus time, showing test results for aconventional metal can and for a container in accordance with anembodiment of the invention;

FIG. 9 is a chart of finite element analysis results for a container inaccordance with an embodiment of the invention, showing criticalpressure (defined as the pressure above which permanent deformation ofthe container body occurs) as a function of retort temperature;

FIG. 10 is a graph of differential pressure and retort temperatureversus time, showing test results for a container in accordance with anembodiment of the invention, wherein the differential pressure wasallowed to increase until a failure of the container occurred; and

FIG. 11 is a cross-sectional view of a container having a double-seamedand sealed end in accordance with another embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention now will be described more fully hereinafter withreference to the accompanying drawings in which some but not allembodiments of the inventions are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout. Thedrawings are not necessarily to scale, and thus the relative proportionsof various elements (e.g., thicknesses of layers in multi-layerstructures) suggested by the drawings is not necessarily indicative ofthe actual relative proportions.

With reference to FIG. 1, several steps of a process for makingcontainers in accordance with an embodiment of the invention areschematically depicted. In a first step, an extruder 10 is employed toextrude a substantially thermoplastic tube 12 as a continuous extrusion.The extruder 10 includes a screw 14 or the like that feeds a moltensubstantially thermoplastic material under pressure through a die 16such that the continuous tube 12 is extruded through an annular dieorifice 18. The extruded tube 12 can have a monolayer or multi-layerconstruction. As an example of a multi-layer construction, the tube wallcan have the structure (from ID to OD): heat-sealable layer/tielayer/barrier layer/tie layer/heat-sealable layer.

The continuously extruded tube 12 is cooled sufficiently (via knowncooling means, not illustrated) and is then cut into parent tubes 20 ofa convenient length. Typically each parent tube 20 will be of sufficientlength to provide a plurality of container bodies 22 cut from the parenttube as shown. Each container body 22 is then mated with a pair of metalends 30.

The metal end 30 and container body 22 in some embodiments can beconstructed to mate with each other as described in Applicant'sco-pending application Ser. No. 13/161,713 filed on Jun. 16, 2011, theentire disclosure of which is hereby incorporated herein by reference.

The metal end 30 includes a central portion 32 and an outer peripheralportion 34 extending generally radially outwardly from the centralportion 32 and extending circumferentially about the central portion 32.The peripheral portion 34 has a radially outer part and a radially innerpart. The radially outer part defines a curl 36 having a lower surfacethat is generally concave downward in an axial direction of the metalend. The radially inner part defines a chuck wall 38 that extendsgenerally downward and radially inward from the curl 36. The chuck wall38 can be a compound-angle chuck wall, as described in the above-noted'713 application, having an upper part adjacent the curl 36 and a lowerpart joined to and positioned below the upper part. The upper part ofthe chuck wall is substantially linear and oriented relative to theaxial direction at a relatively smaller non-zero angle and the lowerpart of the chuck wall is substantially linear and oriented relative tothe axial direction at a relatively larger angle compared to the upperpart of the chuck wall.

The metal end 30 is configured such that at least a bottom edge of thelower part of the chuck wall has an outside diameter that is smallerthan the inside diameter of the container body side wall 24 at the upperedge thereof. Additionally, the chuck wall 38 is configured such that itbecomes somewhat larger in diameter than the inside diameter of thecontainer body side wall 24 as the top edge of the side wall progressesup toward the curl 36 during mate-up of the metal end 30 with thecontainer body 22. In other words, the side wall's ID is undersized inrelation to the OD of the chuck wall adjacent the curl. This has theeffect of “wiping” the inner surface of the side wall 24 with the metalend during mate-up, which has the benefit of cleaning the inner surfaceprior to seaming. This also results in an interference fit between thechuck wall 38 and the side wall 24.

Once the metal end 30 is mated with the container body 22, a seamingoperation is performed in order to seam the metal end onto the containerbody. In the illustrated embodiment, the container body is astraight-walled (non-flanged) container body, and a crimp seam 40 isformed between the metal end and the container body, in which the sidewall 24 remains substantially straight and is compressed between thechuck wall 38 and a deformed portion of the curl 36. Alternatively, inother embodiments, a double seam can be formed (see, for example, FIG.11), in which case the container body can be flanged. The crimp seam 40has the advantage of being usable with non-flanged container bodies andyet providing a positive locking of the metal end 30 onto the containerbody 22 even before the metal end is heat-sealed to the container body.This can be seen in FIG. 2, which is a photomicrograph of a sectionedcontainer in the region of the crimp seam 40. A “nub” or interlockingportion of the container body side wall is formed by the foldedperipheral edge of the curl “biting” into the side wall. The nub and thefolded edge effectively interlock, thereby locking the metal end ontothe container body.

It will be understood, of course, that a second metal end is attached tothe opposite end of the container body 22 in the same fashion describedabove. Alternatively, in the case of a container body having an integralbottom wall (as may be the case with, for example, a blow-molded,thermoformed, or injection-molded container body), the second metal endis not required.

The above-described interlocking of the metal end 30 and container body22 alone, however, is not sufficient to enable the container towithstand a retort process. In order to be able to withstand retortintact, the container is subjected to a heat-sealing operation to fuseportions of the metal end 30 to the container body side wall 24. In thisregard, at least one of the respective surfaces of the metal end andside wall that are intimately contacting each other in the region of thecrimp seam 40 is formed by a heat-sealable material, and the twosurfaces are such that heating of the crimp seam to soften or melt thisheat-sealable material, followed by cooling of the material, causes thetwo surfaces to be “thermally fused” to each other. More specifically,it is important to the attainment of adequate “blow-off resistance”during retort (or other high-internal-pressure condition of thecontainer) that at least the chuck wall 38 of the metal end 30 bethermally fused to the inner surface of the side wall 24 of thecontainer body, and preferably both the chuck wall 38 should bethermally fused at the ID and a portion of the curl 36 (or, moreaccurately, what was the curl prior to the seaming operation) should bethermally fused at the OD of the container body side wall 24.

The thermal fusing operation is diagrammatically depicted in FIG. 3, andthe resulting thermally fused seam 40 is shown, greatly enlarged, inFIG. 4. As indicated in FIG. 3, the thermal fusing operation can beaccomplished using an induction heater 50. The induction heater 50includes a coil through which a high-frequency alternating current ispassed, thereby creating a high-frequency alternating electromagneticfield. The metal layer of the end 30 is exposed to this alternatingelectromagnetic field, which induces eddy currents (also called Foucaultcurrents) within the metal so as to cause Joule heating because of theresistance of the metal. This heating of the metal then causes heattransfer by conduction to anything in contact with the metal, includingthe heat-sealable material(s) on the end 30 and/or side wall 24.

Thus, as an example, FIG. 4 illustrates the metal end 30 as having ametal layer 42 and an interior layer or coating of a heat-sealablematerial 44. Any suitable heat-sealable material can be used for thelayer 44, non-limiting examples of which include: acrylonitrilebutadiene styrene (ABS), acrylic (PMMA), celluloid, cellulose acetate,cyclic olefin copolymer (COC), ethylene-vinyl acetate (EVA), ethylenevinyl alcohol (EVOH), fluoroplastics (PTFE, alongside with FEP, PFA,CTFE, ECTFE, ETFE), ionomers, liquid crystal polymer (LCP),polyoxymethylene (POM or acetal), polyacrylates (acrylic),polyacrylonitrile (PAN or acrylonitrile), polyamide (PA or Nylon),polyamide-imide (PAI), polyaryletherketone (PAEK or Ketone),polybutadiene (PBD), polybutylene (PB), polybutylene terephthalate(PBT), polycaprolactone (PCL), polychlorotrifluoroethylene (PCTFE),polyethylene naphthalate (PEN), polyethylene terephthalate (PET),polycyclohexylene dimethylene terephthalate (PCT), polycarbonate (PC),polyhydroxyalkanoates (PHAs), polyketone (PK), polyester, polyethylene(PE), polyetheretherketone (PEEK), polyetherketoneketone (PEKK),polyetherimide (PEI), polyethersulfone (PES), chlorinated polyethylene(CPE), polyimide (PI), polylactic acid (PLA), polymethylpentene (PMP),polyphenylene oxide (PPO), polyphenylene sulfide (PPS), polyphthalamide(PPA), polypropylene (PP), polystyrene (PS), polysulfone (PSU),polytrimethylene terephthalate (PTT), polyurethane (PU), polyvinylacetate (PVA), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC),styrene-acrylonitrile (SAN). Where the container is to beretort-processed, a suitable heat-sealable material able to withstandthe retort-processing conditions should be selected.

When the metal layer 42 is heated by induction heating, theheat-sealable layer 44 is heated by conduction, which causes theheat-sealable material to be softened or melted. Because theelectromagnetic field's strength obeys the inverse square law, Jouleheating of the metal end is greatest in the parts of the end closest tothe coil of the induction heater and decreases proportional to theinverse square of the distance from the coil. Thus, only localizedheating of the metal end occurs with a great enough magnitude to causemelting of the heat-sealable layer 44. More particularly, the melting ofthe heat-sealable layer 44 is confined essentially to the region of theseam 40.

As FIG. 4 indicates, the induction heating of the seam 40, followed bycooling (which occurs rapidly upon cessation of the electromagneticfield or movement of the container away from the coil), results in twoareas of thermal fusing between the metal end 30 and container body sidewall 24: there is an inner seal S_(i) between the inner surface of theside wall 24 and a portion of the chuck wall 38 that lies parallel toand intimately contacts the side wall, and there is an outer seal S_(o)between the outer surface of the side wall 24 and a portion of what wasthe curl of the metal end prior to seaming. The seals S_(i) and S_(o) inFIG. 4 (and in FIGS. 5 and 6) are depicted for illustrative purposes asif they were each a distinct layer between the metal end 30 and the sidewall 24, but it is to be understood that in reality the seals are formedby a melding of the heat-sealable layer 44 of the metal end and thethermoplastic material on the surface of the side wall 24 (or, in thecase of FIG. 5 where the metal end does not have a heat-sealable layer,by a fusing of the thermoplastic surfaces of the side wall 24 to themetal end).

It is important to the attainment of adequate blow-off resistance thatthe chuck wall 38 include a portion that is parallel to and intimatelycontacting the inner surface of the side wall 24, and that this portionbe thermally fused as described above. This results in the interfacebetween the chuck wall 38 and the side wall 24 being oriented along adirection substantially parallel to the axis of the container, such thatstress on the interface caused by internal pressure inside the containerexerted on the metal end 30 is predominantly shear stress in the planeof the interface (as opposed to out-of-plane stress tending to peel onepart from the other).

It is also a feature of the present invention that during the heatingstep for thermally fusing the end 30 to the side wall 24, there is asubstantial absence of external pressure exerted on the chuck wall 38and side wall 24. Rather, pressure between the chuck wall and side wallcomes from the interference fit that exists between them, as previouslydescribed. Indeed, when an induction heater 50 is employed, it ispossible for there to be no contact between the heating element and themetal end (although it may be advantageous or desirable to provide sometype of contact with the container, such as for conveying it along apath beneath the induction heating element, when the heating step iscarried out in a continuous conveyor-type process).

Various constructions of the metal end 30 and container body side wall24 can be employed in the practice of the present invention. As notedwith respect to FIG. 4, in one embodiment the metal end 30 can have aninterior heat-sealable layer 44. In this case, the container body sidewall 24 can be a mono-layer construction as illustrated, thesubstantially thermoplastic side wall 24 being heat-sealable to theheat-sealable layer 44 of the metal end.

Alternatively, in other embodiments, the side wall 24 can be amulti-layer construction. For example, the side wall 24 can comprise atleast two layers including an interior heat-sealable layer and a barrierlayer providing moisture and gas barrier properties for the containerbody. The metal end 30 furthermore does not necessarily have to have aninterior heat sealable layer, as long as the interior surface is fusibleto the heat-sealable layer of the side wall 24. FIG. 5 illustrates apossible embodiment along these lines. The metal end 30 does not includean interior heat-sealable layer but rather has a bare metal surface onits interior side. It is illustrated as having a metal layer 42 ofhomogeneous construction, but it is also possible for the metal end tobe, for example, ETP (electrolytic tin plate steel) consisting of alayer of steel to which an ultra-thin coating of tin is electrolyticallydeposited, for example on the interior product-facing surface. Thecontainer body side wall 24 consists of five layers, in order from ID toOD: an interior heat-sealable layer 25, a tie layer 26, a barrier layer27, a tie layer 28, and an exterior heat-sealable layer 29. Any of thepreviously described heat-sealable materials can be used for theheat-sealable layers 25 and 29. The barrier layer 27 can comprise anysuitable material providing the necessary barrier properties for theparticular application to which the container will be put. Non-limitingexamples of such barrier materials include ethylene vinyl alcohol(EVOH), polyvinyl alcohol (PVOH), polyvinylidene chloride copolymer(PVDC), polyacrylonitrile (PAN), polyethylene terephthalate (PET),polyethylene naphthalate (PEN), liquid crystal polymers (LCP), amorphousnylon, nylon 6, nylon 66, nylon-MXD6, and the like. The tie layers 26and 28 can be any suitable adhesive materials for adhering theheat-sealable layers 25 and 29 to the barrier layer 27.

When the metal end 30 does not include a heat-sealable layer, theheat-sealable layers 25 and 29 can be designed to thermally fuse to thebare metal surface so as to form the seals S_(i) and S_(o). For example,an ionomer (e.g., SURLYN® or the like) will thermally fuse to a baremetal such as ETP.

An advantage of the seal structure shown in FIG. 5 is that the innerseal S_(i) and outer seal S_(o) together serve to isolate the barrierlayer 27 of the side wall from moisture migrating either from inside thecontainer or from outside the container and contacting the barrier layer27. This is important when the barrier layer is a material whose barrierproperties are degraded by exposure to moisture. For example, EVOH is anexcellent oxygen barrier, but it is well known that EVOH isdeleteriously affected by moisture. The usual solution would be to burythe EVOH layer between two layers that are good moisture barriers (e.g.,polypropylene). In the container of the present invention, however, thismay not completely protect the EVOH layer because the EVOH layer isexposed at the end surface of the container body side wall. If moisturewere able to migrate into the seam region, it could then migrate intothe EVOH layer through the end surface. The seal structure such as shownin FIG. 5 prevents the end surface of the barrier layer 27 from beingexposed to moisture.

FIG. 6 shows yet another embodiment, in which the metal end 30 has ametal layer 42, an interior coating 44, and an exterior coating 46. Theside wall 24 has a construction similar to that of FIG. 5. The interiorcoating 44 comprises a heat-sealable material that is compatible with(i.e., heat-sealable to) the interior layer 25 of the side wall 24 toform the seal S_(i) and with the exterior layer 29 to form the sealS_(o). The exterior coating 46 is provided primarily for corrosionresistance so that the visible side of the metal end 30 remainsaesthetically pleasing and does not develop corrosion that could be asource of contamination upon opening of the container. Any of variouscoatings can be used on the metal end, such as polyesters, vinyls,acrylics, alkyds, oleoresins, phenolics, and the like.

The above-described embodiments in FIGS. 4 through 6 are not limiting interms of the particular construction of the metal end 30 and side wall24. The present invention is applicable to and includes any combinationof metal end and side wall constructions in which at least one of theirrespective surfaces that are intimately contacting each other in theregion of the seam 40 is formed by a heat-sealable material, and the twosurfaces are such that heating of the seam to soften or melt thisheat-sealable material, followed by cooling of the material, causes thetwo surfaces to be “thermally fused” to each other. Additionally, aspreviously noted, it is important for at least part of the chuck wall 38to be parallel to and intimately contacting the side wall 24 so that aninterior seal S_(i) is created that is placed predominantly in shear byinternal pressure in the container such as during retort.

Containers in accordance with the present invention were manufacturedand were subjected to testing to determine whether the containers wouldbe capable of going through a typical retort process and remainingintact, i.e., with no failure of the metal end-to-side wall seams andwith no permanent deformation of the container. The containers hadcrimp-seamed and induction sealed ends such as shown in FIG. 5, and hadthe following construction:

-   -   Container body inside diameter=3 inches    -   Container body length=4.4375 inches    -   Container body side wall structure: PP/tie/EVOH/tie/PP    -   Side wall thickness=0.035 inch    -   Metal end structure (from interior to exterior): 40 micron        PP/0.0075 inch TFS tin-free steel)/15 micron PET

The containers were tested in an apparatus shown schematically in FIG.7. The apparatus included a closed chamber C for enclosing a containerunder test. The interior of the chamber was fed with steam via a steampressure regulator SP and with air via an air pressure regulator AP. Thesteam and air supplies were regulated so as to maintain a substantiallyconstant 250° F. temperature in the interior of the chamber. A manuallyoperated pressure regulator MPR was used in some of the tests to allowindependent control over the air pressure within the test containerplaced in the chamber. Wireless pressure and temperature transducers(data acquisition/recording devices) were used to measure pressure andtemperature within the chamber and pressure within the test container.

FIG. 8 shows the results of tests of a container in accordance with theinvention (substantially as shown in FIG. 5) as well as a conventionalmetal can having similar overall dimensions and configuration. Theobjective of this test was to measure the differential pressure (=caninternal pressure—chamber internal pressure) experienced by thecontainers when retorted at a substantially constant 250° F.temperature. In this test series, the manual pressure regulator MPR wasnot used, but rather the pressure inside the container was allowed tonaturally respond to the retort environment in the chamber. The metalcontainer developed a peak differential pressure of about 18.9 psi. Incontrast, the container in accordance with the invention developed apeak differential pressure of only about 6.4 psi, nearly one-third ofthat of the metal container. It is believed that the much lower ΔP forthe plastic-body container was due to the plastic body's ability toexpand radially as a result of the internal pressure, which had theeffect of reducing the ΔP relative to that of the relatively rigid metalcan.

This ability to expand is advantageous in terms of reducing the ΔP andtherefore the stress exerted on the seam between the metal end and theplastic container body. However, if the plastic body were to expand toomuch during retort, it could undergo a permanent deformation, whichcould render the container unsuitable for its intended purpose. To tryto determine whether such permanent deformation could pose a problem, afinite element analysis was performed on the container for two wallthicknesses, 0.026 inch and 0.035 inch, and over a range of temperaturesfrom 70° F. to 265° F. The analysis sought to determine at eachtemperature the critical pressure, which is defined as the differentialpressure above which permanent deformation of the plastic body occurs.The results are plotted in FIG. 9. As expected, the critical pressuredecreases with increasing temperature. At a typical retort temperatureof 250° F., the critical pressure was computed to be 10 psi for the0.026 inch wall thickness and 14 psi for the 0.035 inch wall thickness.This analysis provides confidence that under realistic retort conditionssuch as those in the test summarized in FIG. 8, the ΔP of the containersof the present invention should be well under the critical pressure.

Additional tests of containers with crimp-seamed and sealed ends inaccordance with the invention were conducted in the apparatus of FIG. 7in order to determine how much ΔP would be required under typical retortconditions to cause the containers to fail. As with the previouslydescribed tests, the temperature in the retort chamber was maintainedsubstantially constant at about 250° F. By adjusting the manual pressureregulator MPR periodically, the pressure within the container wasstepped up synchronously with the increasing pressure in the testchamber during “come-up”. Once the temperature in the chamber reached acontrolled 250° F. (having maintained minimal differential pressure tothis point), the pressure in the container was then increasedprogressively until failure occurred, in order to evaluate the sealburst resistance due to positive differential pressure. FIG. 7 shows thetemperature and ΔP versus time for one container. The container failedat a ΔP of about 21.9 psi, and the failure mode was a bursting of theplastic container body. Additional repeat tests were performed withseveral other nominally identical containers. Over the series of tests,the ΔP at failure ranged from 21.8 to 22.4 psi, and the failure mode wasalways a bursting of the plastic container body wall. The metal endsremained attached to the container body.

For comparison purposes, the same type of test was conducted oncontainers made from the same container body and metal end components,but having the ends double-seamed onto the container bodies and withoutinduction sealing of the ends. For that series of tests, the ΔP atfailure ranged from 4.4 to 8.3 psi, and the failure mode was always abreach of the double seam of one of the ends (i.e., the double seam“unrolled” as a result of softening of the plastic side wall at elevatedtemperature and the stress applied to the seam from the internalpressure).

Thus, comparing the performance of the inventive containers withcrimp-seamed and sealed ends on the one hand to that of the containerswith double-seamed and unsealed ends on the other hand, there was anapproximately 250% increase in the ΔP at failure for the inventivecontainer. This dramatic improvement was unexpected and is notcompletely understood. It is theorized, however, that the improvement inseam integrity is due in large part to the thermal fusing of the chuckwall to the inner surface of the container body side wall, which resultsin an interface that experiences almost purely shear stress during ahigh internal-pressure condition such as retort. This interface, whichis very strong in shear, is thought to bear the vast majority of thestress exerted on the metal end, such that the seam itself is notsubjected to great stress.

A further advantage of the container of the invention is its ability toundergo elastic expansion during high internal-pressure conditions suchas retort, and then return substantially to its original configurationwhen the high internal pressure is relieved. As noted, this helpsalleviate internal pressure and, consequently, the stresses exerted onthe chuck wall/side wall interface and the seam. To realize thisadvantage, of course, the container body must be relativelyunconstrained so that it is able to expand radially.

The foregoing description focuses on containers having crimp-seamed andsealed metal ends. As noted, however, the invention is not limited tocrimp seaming. Alternatively, the metal ends can be double seamed andthen sealed via an induction heating or other process. FIG. 11 showssuch a container having a double seam 40′. Apart from the different seamconfiguration, the double-seamed containers are similar to thepreviously described crimp-seamed containers. The double seam 40′ ischaracterized by the upper end of the side wall 24 forming a body hookand the curl of the metal end forming an end hook that is interlockedwith the body hook.

In typical double-seamed containers, a seaming compound is often appliedto the metal end in the region of the curl. The seaming compound flowsduring double seaming so as to fill up any gaps that may exist betweenthe metal end and container body wall in the seam area. Containers inaccordance with the invention can be made either with our withoutconventional seaming compounds.

In the foregoing description and the appended claims, references to thecontainer body being “substantially thermoplastic” or the like mean thatthermoplastic is the majority ingredient of the container body on avolume basis, and furthermore that any non-thermoplastic ingredient(s)does (do) not impair the ability of the container body to be heat-sealedto a metal end or to expand elastically during retort processing aspreviously described. For example, a substantially thermoplasticcontainer body can include non-thermoplastic ingredients such aspigments (e.g., titanium dioxide), dyes, or other additives forimparting visual characteristics (e.g., coloration, opacity, etc.) orother properties not provided by the thermoplastic itself. As anotherexample, a container body of composite construction such aspaper/thermoplastic or metal/thermoplastic would not be “substantiallythermoplastic” (even if the thermoplastic were the majority ingredientby volume) if the paper or metal component impaired the ability of thecontainer body to be heat-sealed to a metal end and/or to expandelastically during retort processing.

Containers in accordance with the invention can provide distinctadvantages over conventional metal retort containers. For example, theinvention enables the option of making the container body 22substantially transparent so that the contents of the container can beseen by the consumer prior to purchase. Particularly for visuallyattractive products (e.g., fruits and vegetables) this can provide aperception of freshness. Alternatively, the container body can be tintedany of various colors while still remaining substantially transparent,or can be made opaque, through incorporation of suitable dyes orpigments in the thermoplastic material. Additionally, the container canbe free of bisphenol-A (BPA). The container can be microwavable, unlikea conventional metal can.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. For example, whileinduction heating has been described for causing the metal end andcontainer side wall to become thermally fused together in the seam area,other types of heating devices and processes can be used instead.Therefore, it is to be understood that the inventions are not to belimited to the specific embodiments disclosed and that modifications andother embodiments are intended to be included within the scope of theappended claims. Although specific terms are employed herein, they areused in a generic and descriptive sense only and not for purposes oflimitation.

What is claimed is:
 1. A method for making a container, comprising thesteps of: providing a container body having a side wall extending abouta container body axis, the side wall having a lower end and an upperend, the upper end defining an upper edge that extends about a topopening of the container body, the side wall having an inner surface andan outer surface; providing a metal end for closing the top opening ofthe container body, the metal end having at least a metal layer andcomprising a central portion and an outer peripheral portion extendinggenerally radially outwardly from the central portion and extendingcircumferentially about the central portion, the peripheral portionhaving a radially outer part and a radially inner part, a firstheat-sealable material being present on one of (a) a lower surface of atleast the peripheral portion of the metal end and (b) the inner surfaceof the side wall adjacent the upper end thereof, the radially outer partof the peripheral portion defining a curl, the radially inner part ofthe peripheral portion defining a chuck wall that extends generallydownward from the curl and has a radially outer surface; applying themetal end to the container body such that the metal end closes the topopening and the radially outer surface of the chuck wall and the innersurface of the side wall have an intimately contacting interfacetherebetween; forming a seam connecting the metal end to the upper endof the side wall, the seam being formed by interlocking the curl of themetal end with the upper end of the side wall via a biting interface ofa folded peripheral edge of the curl and the side wall; after formationof the seam is completed, heating the first heat-sealable material to atemperature sufficient to cause the first heat-sealable material incontact with the radially outer surface of the chuck wall and the innersurface of the side wall to be softened or melted; and allowing thefirst heat-sealable material to cool and harden such that the interfacebetween the chuck wall and the side wall is fused, the interface beingoriented along a direction such that stress on the interface caused byinternal pressure inside the container exerted on the metal end ispredominantly shear stress.
 2. The method of claim 1, wherein the stepof forming a seam comprises forming a crimp seam.
 3. The method of claim1, further comprising providing a second heat-sealable material presenton the other of (a) the lower surface of at least the peripheral portionof the metal end and (b) the inner surface of the side wall adjacent theupper end thereof, and wherein: the second heat-sealable material andthe first heat-sealable material are placed in contact with each otherat the interface between the chuck wall and the side wall, and the firstand second heat-sealable materials are heated to a temperaturesufficient to cause the first and second heat-sealable materials to besoftened or melted and to flow together, after which cooling of thefirst and second heat-sealable materials is allowed to occur so as tofuse the chuck wall to the inner surface of the side wall.
 4. The methodof claim 3, wherein the second heat-sealable material and the firstheat-sealable material are thermally fused together in the seam as well.5. The method of claim 1, further comprising the steps of: filling thecontainer with a food product prior to the step of applying the metalend to the container body; and after the interface between the chuckwall and the side wall is fused, retorting the container.
 6. The methodof claim 5, wherein during the retorting step the container body isradially unconstrained such that the container body is allowed to expandradially as internal pressure is exerted on the side wall, therebyreducing pressure build-up within the container and consequently thestress placed upon the side wall and metal end.
 7. The method of claim6, wherein the container body is free of any special expansion panels,whereby the radial expansion of the container body occurs substantiallyuniformly about a circumference of the container body.
 8. The method ofclaim 1, wherein the chuck wall extends at a non-zero acute anglerelative to a longitudinal axis of the container body and is configuredsuch that a lower end of the chuck wall is smaller in diameter than theinner surface of the side wall, while an upper end of the chuck wall islarger in diameter than the inner surface of the side wall, and whereinthe step of applying the metal end to the container body results in theside wall of the container body moving relatively upward from the lowerend to the upper end of the chuck wall such that an interference fit iscreated between the chuck wall and the side wall, thereby creating saidintimately contacting interface therebetween.
 9. The method of claim 8,wherein during the heating step there is a substantial absence ofexternal pressure exerted on the chuck wall and side wall, pressurebetween the chuck wall and side wall coming rather from saidinterference fit.
 10. A method for packaging and retort-processing afood product, comprising the steps of: providing a container assemblythat includes a container body having a side wall and further includesan end wall closing a lower end of the container body, an opposite upperend of the container body being open; providing a metal end having atleast a metal layer and comprising a central portion and an outerperipheral portion extending generally radially outwardly from thecentral portion and extending circumferentially about the centralportion, the peripheral portion having a curl and a chuck wall thatextends generally downward and radially inwardly from the curl;providing at least one heat-sealable material on at least one of (a) alower surface of the peripheral portion of the metal end and (b) aninner surface of the container body adjacent the upper end thereof;placing the food product into the container assembly through the openend of the container body; forming a crimp seam between the metal endonto the container body via a biting interface of a folded peripheraledge to close the open end thereof, the forming step causing the sidewall of the container body to be compressed between the chuck wall on aninner side of the side wall and a permanently deformed portion of themetal end formed by deforming the curl on an outer side of the sidewall; thermally fusing the metal end to the container body by causingthe heat-sealable material(s) to be melted where the metal endcompresses the side wall and then allowing the melted heat-sealablematerial(s) to cool and harden, thereby completing a filled container;and retort-processing the filled container to sterilize the food productand interior of the container.
 11. The method of claim 10, whereinduring the retort-processing step the container body is radiallyunconstrained such that the container body is allowed to expand radiallyas internal pressure is exerted on the side wall, thereby reducingpressure build-up within the container and consequently the stressplaced upon the side wall and metal end.